The present disclosure concerns fluidfoils, gas turbine engines and methods of managing flow tip momentum transfer. More specifically the disclosure concerns, in part, particular fluidfoil fence arrangements. The disclosure may have particular utility when applied to some or all of the propeller blades of open rotor gas turbine engines having contra-rotating propeller stages, but this is not intended to be limiting.
It is known to provide the tips of fluidfoils with fences in the form of an aerodynamic fence at the end of the fluidfoil. The fence is leant at a particular lean angle, either towards the suction side of the fluidfoil (dihedral fence) or towards the pressure side of the fluidfoil (anhedral fence). Some fluidfoil tips may also have both an anhedral and a dihedral fence.
Fences increase efficiency by reducing fluid momentum in the radial direction as it passes the fluidfoil.
Fences also serve to reduce vortices at the fluidfoil tips. Vortices are inefficient and can be acoustically undesirable, especially where downstream structures intercept the vortices. A fence inhibits the flow of fluid from the pressure side of the fluidfoil (where the pressure is greater) to the suction side (where the pressure is lower). This tends to limit or prevent the creation of a spiralling trail of fluid (vortex) leaving the fluidfoil tip.
Where however momentum transfer from the pressure side to the suction side is insufficient in view of the presence of a fence, a vortex breakdown occurs and a bubble is formed. Vortex breakdown is an abrupt change of flow structure that occurs in swirling flows. If the fence causes sufficient reduction in momentum transfer, there may be a rapid deceleration of the flow in the axial direction in the vicinity of the fence. Further there may be a rapid expansion of the vortex core at approximately 50% of the tip chord, giving rise to a bubble like structure.
Although the breakdown causes the vortex to be weaker, it does not imply the decay of vorticity itself. The swirling is in fact redistributed over a larger area, potentially worsening the downstream wake. Vortex breakdown tends therefore to be even less desirable than a standard vortex from an acoustic viewpoint, particularly tending to increase broadband noise. Moreover, the overall direction of axial flow is usually reversed inside the bubble, thereby compromising, to some extent, aerodynamic performance.
Unless otherwise specified, the word axial is used throughout this specification to refer to the direction of the main rotational axis of a gas turbine engine to which fluidfoils would, in use, be attached. Similarly the word radial is used with respect to a gas turbine engine to which fluidfoils would, in use, be attached. Specifically radial refers to directions perpendicular to the main rotational axis of the gas turbine engine.